1. Field of the Invention
This invention in general relates to optical devices and in particular to optical devices which are suitable for use as camera viewfinders.
2. Description of the Prior Art
Some of the most important, if not paramount, considerations for commercially successful cameras for use in amateur hand photography are cost and convenience and, as well, simplicity and reliability of operation. Also, cameras intended for use both indoors and outdoors by a relatively untrained individual desirably are lightweight and adapted for carrying or for safe storage when not in use, as on a shelf or in a drawer. These several requirements have been commonly met today by the use of molded plastics both for lenses and camera body parts because the specific gravity of plastics is low in comparison to glass and metallic materials.
For cameras of the class required for instant photography, other considerations enter as the arrangement of camera loading and print ejection and a restricted range on size because of requirements related to optical path volume. More particularly, the optical path volume tends to define basic geometry around which a camera must be designed and built. That is, a camera, whether with a straight or folded optical path, will have this basic size which ultimately must expand to accommodate all of its components such as shutter, viewfinder, film transport mechanism, and so forth. For minimum size and weight then, one task is to minimize those parameters for each component required while still meeting other fundamental needs.
For instant cameras, as in others, the optical path volume, i.e., the space bounded by the extreme bundle of rays surrounding what is to be photographed, is determined by the desired angular field coverage and print diagonal dimension. Print diagonals commonly practiced range in magnitude between 3 and 5 inches, more or less, and thus required objectives have equivalent focal lengths on the order of 115 mm. Thus, one arrives at a basic volume which is approximately a truncated cone shape with a base size related to the print diagonal and an altitude of approximately 115 mm. This, of course, may be folded through the use of mirrors and film loading and ejection arrangements, but otherwise is basically fixed. Consequently, the need can be appreciated for miniaturization of all other camera components wherever possible.
Viewfinders, a necessary component for aiming the camera and giving a visual indication of what is contained in the photograph, are usually either of the single lens reflex type relying in part on the camera objective lens for this purpose or the independent type which functions separate from the camera objective.
The independent types today are usually for aesthetic reasons built into the camera body and employ optical systems derived more or less indirectly from a Galilean telescope. However, the reversed Galilean form of viewfinder becomes less suitable where the length of the viewfinder tends to become large as in the case of instant camera structures. Here, as the separation between divergent and convergent groups increases, power adjustments force the magnification unacceptably downward. Moreover, reversed Galilean forms are afflicted with other problems such as distortion, limited field angle, field shifts and defocused field defining masks.
Other telescopic forms have been used and lengthened through the use of periscopic principles involving the use of field and relay lenses. These too suffer from narrow angular fields and are difficult to correct because of the need for extensive use of positive lenses.
It is thus a primary object of the present invention to provide an optical system suitable for use as a camera viewfinder.
It is another object of the present invention to provide an elongated, but otherwise minimal sized optical viewfinder device.
Yet another object of the present invention is to provide a well-corrected optical device having moderately large angular field coverage with minimum optical elements.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. The invention, accordingly, comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure.